#include <stdio.h>

int main()
{
	int N  = 0;
	scanf( "%d", &N );
	float a_0, a_Np1;
	scanf( "%f\n%f", &a_0, &a_Np1 );

	const int N_MAX = 3000;
	//float c[ N_MAX ];
	double mega_sum = 0;
	for( int i = 0; i < N; ++i )
	{
		//scanf( "%f", &c[i] );
		float tmp;
		scanf( "%f", &tmp );
		mega_sum += ((double)tmp) * (N-i);
	}

	double a1 = ( (double)a_0 * N + (double)a_Np1 - 2 * mega_sum ) / (N+1);

	float res = (float)a1;

	printf( "%.2f", res );

	return 0;
}

